The present invention relates to a reflectance reducing film, and a method of forming the film on a glass substrate. The film-forming method belongs to the sol-gel process. The reflectance reducing film is suitable for application, for example, to architectural window glasses.
Hitherto, there have been various proposals to form a reflectance reducing film on a glass substrate surface.
For example, JP-B-41-666 discloses an etching composition containing hydrofluoric acid for the purpose of providing a glass substrate with a low reflectance. The glass substrate is etched with the etching composition for forming a reflectance reducing film or layer which is lower than the glass substrate in refractive index.
JP-B-2-14300 discloses a method of producing a reflectance reducing glass. In this method, a reflectance-reducing porous layer is formed by selectively removing components of a glass plate except silica. This removal is made by contacting the glass plate with a silica-supersaturated aqueous solution of hydrosilicofluoric acid.
However, the above-mentioned methods of producing a reflectance reducing glass are relatively complicated in production process. Therefore, it is necessary to spend a relatively long time to produce a reflectance reducing glass. Furthermore, in case that a large reflectance-reducing glass is produced, there is provided a problem of increased error in optical characteristics of the glass. Still furthermore, due to the use of hydrofluoric acid which is very hazardous against human body, the production process must be conducted very cautiously. This lowers the production efficiency.
Another example is forming a reflectance reducing film on a glass substrate by applying a metal alkoxide solution to which an organic polymer is added. The coated glass substrate is heated for thermally decomposing the organic polymer, thereby making the film minutely rough. However, this method has the following drawback.
After heating at a temperature not lower than about 400.degree. C., micro-pits which make the film minutely rough tend to disappear by the densification of the film.
Still another example is forming a multilayer film on a glass substrate by alternately arranging at least one film having a high refractive index and at least one film having a low refractive index. However, the forming process is relatively complicated, thereby lowering the production efficiency. Furthermore, the multilayer film has a relatively high reflectance for the light which is obliquely incident on the multilayer film. Still furthermore, the coated glass substrate has a relatively high excitation purity, thereby causing glare.
JP-B-3-23493 discloses a low reflectance glass having a two-layered film formed thereon. The two-layered film comprises a first layer of a condensation product containing a metal oxide and a second layer of a condensation product of a fluorine-containing silicon compound. After forming the first layer on a glass substrate, the first layer is heated at a temperature ranging from 200.degree. to 550.degree. C. After forming the second layer on the first layer, the second layer is heated at a temperature ranging from 120.degree. to 250.degree. C. With this heating temperature of the second layer, the second layer becomes insufficient in hardness and strength.